UTRAN (Universal Terrestrial Radio Access Network) is a term identifying the radio access network of a UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) and NodeBs i.e. radio base stations. The NodeBs communicate wirelessly with mobile user equipments (UEs) and the RNCs control the NodeBs. The RNCs are further connected to the Core Network (CN). Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further, the E-UTRAN consists of radio base stations (eNBs), and the eNBs are interconnected and further connected to the Evolved Packet Core network (EPC). E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP).
The Radio Resource Control Protocol (defined in TS 36.331) is the signaling protocol responsible for configuring and re-configuring lower layers of the UE. These lower layers include the physical layer, Medium Access Control (MAC), Radio Link Control Protocol (RLC), and Packet Data Convergence Protocol (PDCP). RRC is also responsible for configuring and re-configuring e.g. UE measurements, and the RRC protocol is also in control of connected mode mobility. The RRC protocol is terminated in the eNB and the UE, respectively.
The RRC specification includes several functions and procedures. One function is related to the re-configuration of the UE, as illustrated in FIG. 1.
In this procedure, the eNB in the E-UTRAN issues a reconfiguration message transmitted to a UE. Upon successful reception of the message and in case the reconfiguration procedure is successfully completed, the UE reconfigures the parameters and functions indicated in the reconfiguration message, and responds with a compete-message to the eNB. The reconfiguration message could have a large variety of content, including e.g. L1, MAC, RLC, PDCP or measurement parameters. A handover may also be commanded with the same message.
In scheduled transmissions on a shared channel, a UE identity of a scheduled transmission must also be conveyed on an out-band control channel (HS-SCCH in UTRAN downlink, and PDCCH in E-UTRAN) to identify which UE the scheduling command is intended for. In UTRAN and E-UTRAN this identity is not explicitly transmitted, but implicitly included in the CRC calculation and the HS-SCCH channel coding.
The aforementioned identity must be unique for the UE, in case only a single UE is scheduled. In UTRAN, this downlink (HS-DSCH) identity is called HS-RNTI, while uplink (E-DCH) scheduling is based on an E-RNTI (RNTI—Radio Network Temporary Identity). In LTE, the current abbreviation for the unique UE identity is C-RNTI, where “C” reflects that this UE identity is unique for the UE in this cell. An UE may obey scheduling commands associated with several such identities.
In LTE, scheduling is the responsibility of the eNB—both in the uplink and the downlink:                In the downlink (DL), information on the PDCCH is sent in parallel with the data on the DL-SCH, such that the right UE can decode the data correctly.        In the uplink (UL), information on the PDCCH is sent prior to the event when the UE should send its data on UL-SCH, such that the UE can encode and transmit the data correctly.        
To successfully receive the aforementioned data, it is important that the two peers (UE and eNB) have compatible configurations.
In the communication between the UE and the eNB, it is often critical that the two peers have compatible configurations, i.e. that the transmitter and receiver uses compatible ways of communicating on all protocol levels. Therefore, it is important or at least desirable that the UE and eNB takes the new reconfiguration into use at the same time.
It should be noted that the aforementioned RRC reconfiguration message is transmitted over the air interface that is subject to strong variations in link quality such as fast and slow fading resulting in transmission errors. Therefore, the reconfiguration message can be lost or delayed by re-transmissions by lower layer protocols. LTE RLC supports ARQ (automatic repeat request) and LTE MAC supports Hybrid ARQ to recover from such transmission errors.
Thus, it may be difficult for the eNB to exactly know when the UE has taken a new configuration into use. It should also be noted that ARQ and HARQ feedback is subject to transmission errors, why such indications only give a hint of when the UE may have successfully received the RRC connection re-configuration message. Uncertainty of the required UE processing time to complete the reconfiguration procedures indicated in the message will further add to the uncertainty of when the UE is prepared to switch from one configuration to another. This timing uncertainty of when the reconfiguration is competed is illustrated with a dashed box in FIG. 1.
In a particular example, the case when the eNB issues a Layer 1 MIMO reconfiguration is considered, where a UE is currently configured to receive the Physical Downlink Shared Channel (PDSCH) with e.g. transmit diversity, but the desire is to reconfigure PDSCH to spatial multiplexing. Clearly, a UE configured to one of a set of available MIMO schemes will not be able to receive PDSCH if the transmitter (eNB) is using a different scheme. Thus, if the reconfiguration of the PDSCH is asynchronous, there is a risk that the connectivity between the UE and the eNB is lost.
Consider e.g. the case where a HARQ feedback error occurs during the transmission of the DL RRC CONNECTION RECONFIGURATION message containing a critical L1 reconfiguration. A HARQ feedback error denotes the case when the HARQ transmitter misinterprets the feedback received from the receiver.
Assume that a negative acknowledgement (NACK) was interpreted as a positive acknowledgement (ACK) by the eNB. We refer to this error as an NACK-to-ACK error.
In this case, eNB will assume that the UE has successfully received the reconfiguration message and will take the new configuration into use within, say, 10 ms after the reception of the falsely decoded ACK. However, as the eNB then switches to the new configuration, the UE will not be able to receive any data on the reconfigured PDSCH, as the UE did not receive the reconfiguration message. Similarly, the eNB may lack means to receive any UL transmissions in case the reconfiguration included a critical reconfiguration of uplink L1 parameters.
In UTRAN (TS RRC 25.331), the present synchronization problem is solved by an “activation time” (a reference to a certain Connection Frame Number, CFN) that can be included in an RRC message in order to assure that the UE starts using a new configuration exactly at the same time instance as the Node B. This referenced CFN should occur sufficiently far in the future, such that the configuration message can be received by the UE, and the UE has time to issue the required reconfigurations. In a successful operation, the UE and Node B then switch exactly at the same moment to the reconfigured configuration.
Since the RRC message may be subject to radio link control (RLC) retransmissions the activation time must be set far enough into the future to allow for retransmissions of the message. Even if the average retransmission delay is small there are a few percent of the messages that needs several retransmissions. The activation time needs to be set to cover also the worst case which leads to that synchronized re-configuration in UTRAN causes a relatively long delay. This affects the signalling performance.
Thus, there is a need to provide a solution for synchronous reconfigurations in LTE without an activation time, by which the ambiguity with regards to the current configuration in the UE preferably can be avoided.